Gas burner control device with low pressure cutoff

ABSTRACT

A gas burner control device including a biased closed, electromagnetically operated, fuel cutoff valve dependent upon electrical energy generated by a pilot burner-heated thermocouple to hold it open, and a biased open pressure responsive switch controlling the thermocouple-cutoff valve circuit, the switch being dependent upon a predetermined minimum gas supply pressure to close it and hold it closed, whereby either a drop in fuel supply below a predetermined minimum pressure or the extinguishment of the pilot flame effects closure of the fuel cutoff valve.

United States Patent 1 Kinsella et al.

[ Jan. 16, 1973 [54] GAS BURNER CONTROL DEVICE WITH LOW PRESSURE CUTOFF [75] Inventors: Howard R. Kinsella, St. Louis Counmy; John J. Love, St. Louis; Carl A. Smith, Lemay; Charles D. Visos, Manchester, all of Mo.

[73] Assignee: Emerson Electric Co., St. Louis, Mo.

[22] Filed: Aug. 2, 1971 [21] Appl. No.: 168,095

[52] US. Cl. ..43l/53, 431/58, 431/80, 431/89, 236/68 D [51] Int. Cl ..F23q 9/08 [58] Field of Search....43 1/53, 52, 54, 55, 58, 78-80, 431/89; 236/68 D, 15 A [56] References Cited UNITED STATES PATENTS 2,962,093 11/1960 Matthews ..43 1/54 X 2/1968 Mitts ..43l/54X 2/1971 Visos ..236/1SA Primary Examiner-Carroll B. Dority, Jr. Attorney-Charles E. Markham [5 7 ABSTRACT A gas burner control device including a biased closed, electromagnetically operated, fuel cutoff valve dependent upon electrical energy generated by a pilot burner-heated thermocouple to hold it open, and a biased open pressure responsive switch controlling the thermocouple-cutoff valve circuit, the switch being dependent upon a predetermined minimum gas supply pressure to close it and hold it closed, whereby either a drop in fuel supply below a predetermined minimum pressure or the extinguishment of the pilot flame effects closure of the fuel cutoff valve.

10 Claims, 1 Drawing Figure OUTLET 7'0 EVE/YE? GAS BURNER CONTROL DEVICE WITH LOW PRESSURE CUTOFF Currently, most gas burner control devices or systems include a pilot flame heated thermocouple providing electrical energy to hold open a biased closed, electromagnetically operated, safety fuel cutoff valve so that failure of pilot flame will result in loss of electrical energy and closure of the cutoff valveLThese devices or systems also frequently include temperature responsive switches responsive to abnormally high temperatures to break the thermocouple cutoff valve circuit thereby to cut off fuel to the burner to preclude a hazardous overheating condition.

A hazardous condition may also result when the fuel supply pressure falls below that which will sustain combustion at the main burner, or below that pressure which will sustain an adequate igniting flame 'at the pilot burner but is yet sufficient to sustain the heating or prolong the cooling of the thermocouple. Moreover, a momentary interruption of the fuel pressure resulting in the extinguishing of both pilot and main burner flames also presents a hazardous condition due to the time interval required for the thermocouple to cool sufficiently, during which, upon a reinstatement of fuel supply pressure, fuel may flow to the main and pilot burners in the absence of pilot flame to ignite it.

It is an object, therefore, of the present invention to provide means in a gas burner control device to effect the immediate cutoff of fuel to either the main burner or to both main and pilot burners when the gas supply pressure drops below a predetermined minimum value.

A further object is to provide a control device for gas burners having a biased closed, electromagnetically operated, safety cutoff valve controlling fuel flow to the main burner and an energizing circuit therefor to hold it open including as a power source a thermocouple heated by a pilot flame, which device includes a pressure operated switch responsive to fuel pressure to maintain completion of the thermocouple electromagnetic valve energizing circuit, thereby to hold open the safety cutoff valve when gas pressure is adequate to sustain combustion at the main burner and to break the energizing circuit and effect instant closure of the safety cutoff valve when gas pressure drops below that which is adequate to reliably sustain combustion at the main burner.

A further object is to provide a control device as in the foregoing paragraph having both main and pilot burner fuel passageways formed therein and in which the safety fuel cutoff valve controls the flow of fuel through both main and pilot burner fuel passageways, whereby fuel to both main and pilot burners is instantly cut off when the fuel supply pressure drops below a predetermined minimum value.

A further object is to provide a control device for gas burners, as in the foregoing paragraphs, which further includes pressure regulating means, and in which the pressure responsive switch is responsive to fuel pressure downstream from said pressure regulating means.

A further object is to provide a control device for gas burners as in the foregoing paragraphs, in which the pressure responsive switch is responsive to the fuel pressure in the pilot burner fuel passageway.

Further objects and advantages will appear from the following complete description of a preferred form of the invention when read in connection with the accompanying drawing.

The single FIGURE of the drawing is a cross-sectional view of a gas burner control device constructed in accordance with the present invention.

Referring to the drawing, the device comprises a body generally indicated at 10, formed of inlet and outlet sections 16 and 28, respectively, joined along the line 12 and connected by screws 14. The inlet body section 16 houses a hollow tapered plug valve 18, a dual purpose pressure regulating and safety cutoff valve 20, and an electromagnet 22 energized by a thermocouple 24 positioned so as to be impinged by the flame of a pilot burner 26. The outlet body section 28 houses a thermostatically operated main valve 30 and a clicker disc 32 to effect its snap action operation. A mounting flange 34 having a threaded stud 36 is attached to the outlet body section 28 by screws 38, and a rod and tube type thermostatic actuator 40 supported in the mounting flange 34 is operative to actuate the valve 30 through a lever 33, an axially slidable cylindrical member 35, and the clicker disc 32.

The hollow plug valve 18 has an intermediate tapered portion seated in a tapered bore 42 in the inlet body section 16, an upper cylindrical flanged portion 44 loosely fitting in an upper concentric cylindrical bore 46, and a lower cylindrical portion 48 fitting a lower concentric cylindrical bore 50. The electromagnet 22 is entered into the bore 50 through a smaller concentric bore and is provided with a lower screwthreaded portion 54 threadedly engaged in a screwthreaded portion of the smaller bore.

The inlet body section 16 has a horizontal inlet passage 56 intersecting the lower cylindrical bore 50, which passage is screw threaded for receiving a gas supply conduit, and a horizontal communicating passage 58 connecting tapered bore 42 with concentric, cylindrical, main valve chambers 60 and 62 formed in the outlet body section 28. The valve chamber 62 is smaller in diameter than the valve chamber 60, whereby a shoulder forming a valve seat 61 is provided for thermostatically operated valve 30. The valve chamber 62 is intersected by a vertical outlet passage 64, the lower end of which is screw threaded for connection of a fuel conduit leading to a main burner (not shown).

The hollow rotary plug valve 18 is provided with ports 66 in the lower cylindrical portion 48 thereof, which ports register with inlet passage 56 to provide communication at all times between the inlet passage and the interior of the hollow plug valve. The plug valve 18 is also provided with a main burner port 68 in the tapered portion thereof which registers with passage 58 to complete communication between the inlet passage 56 and valve chamber 60 when the plug valve is rotated to an on" position. The plug valve 18 is further provided with a circumferentially elongated pilot gas port 70 in the tapered portion thereof which registers with a passage 72 leading from the port 70 to a screw-threaded pilot outlet 73. A conduit 74 connects pilot outlet 73 with pilot burner 26. The circumferential extent of pilot port 70 permits maintaining registry of the port 70 with the passage 72 to supply fuel to the pilot burner when the plug valve 18 is either rotatably positioned to permit flow from port 68 to the main burner or in a position to cut off flow through port 68 to the main burner.

The lower hollow cylindrical portion 48 of plug valve 18 is provided with a valve seat 76 at the upper end thereof with which the dual purpose cutoff and regulating valve cooperates to control the flow through the tapered portion of the hollow plug valve 18. The poppet-type valve 20, being constructed of rubberlike material, is mounted on a valve stem 78, which stem extends vertically above and below the valve 20. At its upper end valve stem 78 is connected to the central portion of a transverse flexible diaphragm 80. The periphery of diaphragm 80 is clamped between the flange 44 at the upper end of hollow plug valve 18 and the rim of a cap member 82 which forms a closure at the upper end of the hollow plug valve. The cap member 82 is rigidly connected to the top of plug valve 18 by suitable means (not shown). The diaphragm 80 has a rigid cup member 84 bonded to a central portion thereof, thereby to provide vertical space for a relatively long, soft spring 86 which biases the valve 20 toward an open position.

At its lower end the valve stem 78 is connected to the central portion of a flexible diaphragm 88. The periphery of diaphragm 88 is fixed to the wall of the lower hollow cylindrical portion 48 of the plug valve by a press-fitted bushing 89. The ports 66 in the wall of the lower hollow cylindrical plug valve portion 48 lie between the valve seat 76 and diaphragm 88, so that inlet gas pressure acts downwardly on diaphragm 88, as well as upwardly against valve 20, thereby to substantially balance the valve with respect to inlet pressure.

A centrally perforated plate 90 fixed to the body section 16 at the upper end of cylindrical bore 46 forms a closure, and a compression spring 92 positioned between the plate 90 and cap member 82 biases hollow plug valve 18 in firmly seated position in tapered bore 42. The cap member 82 has a central perforation and circularly arranged, upwardly extending prongs 83. An operating knob 94 extending through the central perforation in closure plate 90 is provided with deep, axially extending, circularly arranged recesses 96 extending upward from the lower end thereof. The recesses 96 slidably receive the circularly spaced prongs 83 on the cap member 82. The knob 94 is thereby axially movable on the plug valve 18, but rotates with the plug valve.

The operating knob 94 is provided with a retaining flange 98 at its lower end, and an external rib 100 extending radially from the side of the knob registers with a slot 102 in the closure plate 90 to permit the downward vertical movement of the operating knob only when the plug valve is rotated to a "pilot" position. The operating knob 94 is further provided with a screw threaded axial bore 104 having a short, hollow, headless adjusting screw 106 threadedly engaged therein and a headed screw 108 closing the upper end of the bore. The spring 86 bears at its upper end against adjusting screw 106 and at its lower end against cup member 84, thereby biasing the operating knob 94 upward and the central portion of diaphragm 80, the valve stem 78, and, consequently, valve 20 and pressure balancing diaphragm 88 downward in a valve opening direction. Attached to the closure screw 108 is a long, small diameter, actuating rod 110 extending downward through hollow adjusting screw 106 and the spring 86 to a point just short of engaging diaphragm 80 when the valve 20 is completely closed on its seat 76.

The electromagnet 22 has an electromagnetic winding, an iron core, an axially movable armature (not shown) enclosed within a casing 23, and a rod 1 12 connected at its lower interior end to the armature and extending exteriorly upward from the end of the casing. The rod 112 carries a disc 114 on the upper end thereof, and a spring 116 bearing against the upper end of casing 23 and the lower surface of disc 114 biases the armature upward in a direction away from the magnetic core. The actuating rod 1 10 is of such length as to engage the upper end of the valve stem and diaphragm assembly and cause valve 20 to be opened and the armature of the electromagnet to be pressed against the iron core when the operating knob 94 is fully depressed in a pilot position.

The electromagnetic winding within casing 23 is sufficiently energized by electrical energy generated at the thermocouple junction 24 through coaxial conductors 1 18 to hold the armature against the iron core when the junction is heated by the flame of the pilot burner 26. The electromagnet is shown in an energized, retracted position, in which position the valve 20 is free to move between open and closed positions in response to pressure variation downstream thereof. in the absence of pilot burner flame, the electromagnetic winding becomes de-energized and the spring 1 16 moves the armature away from the core and the disc 114 upward into engagement with the lower end of valve stem 78 and the diaphragm 88, and the spring 1 16 has sufficient length and strength to close the valve 20 firmly on its seat 76 thereby to cut off all fuel flow through the device.

The rod and tube type thermostat 40 comprises a tube 41 having a closed outer end and an inner end threadedly engaged in a screw-threaded bore in the mounting stud 36 and a coaxial rod 43 fixed at its outer end to the closed outer end of tube 41 and at its inner end engaging the lever 33 at an intermediate point thereon. The tube 41 has a relatively high coefficient of thermal expansion while the rod 43 has a relatively low coefficient so that a change in the temperature ambient to the tube 41 effects an axial movement of the rod 43. The lever 33 is fulcrumed at one end on the end of an adjusting screw 120 and at its other end bears against the outer end of cylindrical member 35. The cylindrical member 35 is slidably mounted in a bore 122 and is provided on its inner face with an annular knife edge 124 which engages the clicker disc 32 near its periphery.

inward pressure on cylindrical member 35 causes clicker disc 32 to snap through a planar shape from the outwardly convex form shown to an outwardly concave form. in this movement, the disc engages a valve stem 31 connected to main valve 30 and opens the valve against the spring 29. When the inward pressure on cylindrical member 35 is released, the spring 29 initiates the return of the clicker disc and firmly closes the valve 30.

The upper side of pressure regulator diaphragm communicates with atmosphere through a vent 81 in the cap member 82, and the lower side of the pressure balancing diaphragm 88 communicates with downstream pressure through passageways 126 and 127 extending from the lower side of diaphragm 88 to the pilot passageway 72. The lower side of diaphragm 88 is thereby in communication with downstream pressure when only the pilot port 70 is open in pilot position as well as when both ports 70 and 68 are open in on position.

A cavity 128 having a circular side wall is formed in the lower surface of the inlet body portion 16, which cavity receives a pressure operated switch generally indicated at 130. Switch 130 comprises a rigid circular cup-shaped member 132 formed of dielectric material, a flexible circular diaphragm member 134 also formed of dielectric material, a pair of stationary contacts 136 and a conductive bridging element 138 which when in the position shown in the drawing effects conduction between contacts 136. The conductive bridging element 138 has an upwardly extending stem portion encased in a central boss portion of flexible diaphragm 134 and therefore moves with the central portion of the diaphragm.

The cavity 128 is provided with a counterbore 140 forming an annular shoulder 142. Peripheral portions of the cup member 132 and diaphragm member 134 are received in the counterbore with the peripheral portion of the diaphragm lying against shoulder 142 and the peripheral portion of the cup member overlying the diaphragm member. The peripheral portion of the rigid cup member is pressed against the diaphragm member and shoulder and held firmly fixed by staking; that is, by deforming portions of the body member adjacent the periphery of the cavity 128, as indicated at 144, so as to overlie the peripheral portion of the cup member.

The central portion of diaphragm 134 and bridging member 138 are biased upward away from contacts 136 by a spring 146. The contacts 136 include hollow terminal connector portions 148 extending downward and exteriorly of the cup member which receive and connect thereto the ends of the interior one of the coaxial thermocouple leads 118. The switch therefore controls the energizing circuit for the electromagnet 22. The cavity 128 communicates with the pilot burner fuel passageway 72 via passage 126, and the pressure operated switch 130 is therefore responsive to supply pressure as regulated by valve when the plug valve 18 is in either pilot or on position.

OPERATION The elements of the device are shown in the positions they assume when the tapered plug valve is rotated to an on position and the pilot burner is burning, but the main burner is not burning because the temperature of the medium being heated by the main burner is at or near the temperature selected to be maintained and therefore requires no more heating.

Under these conditions, when sufficient supply pressure exists, the electromagnet 22 is energized, thereby holding the actuator rod 112 and disc 114 in retracted position. Both ports 68 and 70 in the plug valve are open, but main valve is closed. Fuel flowing to the pilot burner 26 through inlet 56, ports 66, valve seat 76, port 70, passage 72, and conduit 74 is regulated at a pressure preselected by adjustment of the spring 86. As the pressure on the downstream side of valve 20 tends to increase above the preselected pressure due to higher inlet pressure, the diaphragm 80 moves the valve toward a closed position, and when the pressure on the downstream side of valve 20 decreases due to lower inlet pressure, the valve 20 is moved openward by spring 86.

A predetermined drop in temperature of the medium being heated below that preselected to be maintained will cause the rod and tube thermostat 40, immersed in the medium, to effect a snap action opening of main valve 30, and fuel now flows through the main burner outlet to the main burner where it is ignited. The fuel now flowing to both the pilot and main burners through the upper portion of the hollow plug valve 18, the ports and 68, and passages 72 and 58, respectively, is regulated by the single valve 20 and its operating diaphragm 80.

The effective area of pressure balancing diaphragm 88 is preferably made such as to effect substantially the same downward force on the valve stem 78 as the upward force applied thereto by valve 20 when in a closed or near closed position due to inlet pressure acting on these two elements. This provision permits the operating diaphragm 80 working between downstream pressure, the reference atmospheric pressure, and the adjusted tension of spring 86 to closely maintain the preselected downstream pressure. Without this provision the downstream pressure would increase somewhat above the preselected pressure and drop somewhat below the preselected downstream pressure as the inlet pressure drops or increases, respectively. By connecting the downstream side of balancing diaphragm 88 with pilot burner passage 72, the diaphragm is rendered operative when the plug valve 18 is in pilot position as well as in on position when the valve 30 is open.

When the medium to which thermostat 40 is responsive is sufficiently heated, the main valve 30 will close, thereby cutting off fuel to the main burner. The valve 30 will thereafter continue to open and close to cycle the main burner off and on to maintain a temperature of the medium which may be selected by turning the adjustable pivot screw 120.

When it is desired to terminate operation of both main and pilot burners and cut off all fuel supply thereto, the manual knob 94 is turned to rotate the plug valve to an off position in which both ports 70 and 68 are out of registry with respective passages 72 and 58. When this occurs, the pilot burner will be extinguished, the thermocouple junction 24 will cool, and the electromagnet 22 will be de-energized. De-energization of electromagnet 22 permits spring 116 to move rod 112 upward and disc 114 into engagement with the diaphragm 88 and the lower end of valve stem 78 and to move the valve 20 in firm closure seating engagement with valve seat 76. The device is now in an inoperative condition.

When subsequently it is desired to operate the pilot and main burners, the knob 94 is first turned to pilot position. In this position a portion of the circumferentially extending port 70 registers with pilot passage 72, but port 68 is out of registry with passage 58. Also, in pilot position, the radial rib on knob 94 registers with the radial slot 102 in cover plate 90, thereby to permit axial downward movement of the knob and the actuating pin carried thereby. The knob 94 is now fully depressed manually, causing the valve 20 to be opened and disc 114 and rod 112 to be depressed and the electromagnet armature to be pressed against its iron core or pole piece. Fuel will now flow to pilot burner 26, which is now ignited by any suitable means. During and after ignition of the pilot burner, the knob 94 is held in a depressed position until sufficient time has elapsed for the pilot burner to sufficiently heat the thermocouple 24 and effect energization of the electromagnet. When the electromagnet is energized, it will hold the armature against its core and therefore the rod 112 and disc [14 in retracted position. The knob 94 may now be released, whence it is returned to its position shown by spring 86 and then rotated to an on position wherein both ports 70 and 68 are in registry with their respective passages 72 and 58.

If the knob 94 is released prematurely before the thermocouple 24 is sufficiently heated by the pilot burner, or if insufficient supply pressure exists in passageway 72, the knob will return to its upper position and valve 20 will close under bias of spring 114. Also, if at any time after the thermocouple has been sufficiently heated the pilot burner is extinguished for any reason or if the pressure in pilot passageway 72 drops below a predetermined minimum, the spring 116 will effect the closure of valve 20 and cut off all flow through the device.

The illustrated device is adapted to use as a fuel flow control device for gas-fired water heaters because of the provision of the screw-threaded mounting stud for threaded engagement in the wall of a water tank and because of the provision of a rod and tube type thermostat which is well adapted to immersion in the water to be heated. It will be understood, however, that the device will serve just as well in connection with space heaters and that any temperature responsive means suited to other uses, with suitable connecting actuating mechanism, may be employed to effect the on and off cycling of main valve 30.

We claim:

1. In a fuel flow control device for gas burners, a body having an inlet, an outlet, and means forming a connecting passageway, a pilot burner, a biased closed safety cutoff valve controlling said passageway, manual means for opening said cutoff valve, electromagnetic means operative when energized to hold said cutoff valve open, an energizing circuit for said electromagnetic means including as a power source a thermocouple junction heated by said pilot burner, and a pressure responsive switch controlling said energizing circuit, said switch being responsive to gas pressure in said passageway and operative to complete said energizing circuit when the gas pressure is above a predetermined minimum and to break said energizing circuit when the gas pressure drops below said predetermined minimum.

2. In a fuel flow control device for gas burners, a body having a valve chamber, an inlet and an inlet fuel passageway leading to said valve chamber, a main.

burner outlet, a pilot burner outlet, and main and pilot burner fuel passageways leading from said valve chamber to said outlets, a manual valve in said valve chamber controlling the flow through said main and pilot burner passageways, a biased closed safety cutoff valve in said valve chamber, manual means for opening said cutoff valve, electromagnetic means for holding said cutoff valve open, an energizing circuit for said electromagnetic means including a thermocouple junction heated by said pilot burner, and a pressure responsive switch controlling said energizing circuit, said switch being responsive to gas pressure in said pilot burner fuel passageway to complete said energizing circuit when the gas pressure is above a predetermined minimum and to break said energizing circuit when the pressure drops below said predetermined minimum.

3. The fuel flow control device claimed in claim 2 in which the manual valve is operative to selectively permit the flow of fuel through said main and pilot burner fuel passageways.

4. The fuel flow control device claimed in claim 3 which further includes a thermostatically actuated valve controlling said main burner fuel passageway.

5. The fuel flow control device claimed in claim 2 in which said safety cutoff valve is positioned upstream from said manual valve.

6. In a burner control system, a main burner, a pilot burner, a source of fluid fuel under pressure, thermostatic valve means for controlling the flow of fuel to the main burner, manual valve means for controlling the flow of fuel to both main and pilot burners, a biased closed safety cutoff valve controlling the flow to both main and pilot burners, manual means for opening said safety cutoff valve, means operative in response to flame at said pilot burner to hold said safety valve open, and pressure responsive means operative in response to a drop in fuel supply pressure below a predetermined minimum to render said safety cutoff valve holding means inoperative.

7. The burner control system claimed in claim 6 which further includes pressure regulating means and in which said fuel pressure responsive means is responsive to the fuel pressure as regulated by said pressure regulating means.

8'. In a fuel flow control device for gas burners, a main burner and a pilot burner for igniting the main burner, a body member, a fuel inlet, a pilot burner outlet, and a main burner outlet in said body, said body being further provided with a pilot fuel passageway leading to said pilot burner outlet, a main burner fuel passageway leading to said main burner outlet and means forming a common fuel passageway connecting said inlet with said pilot and main burner fuel passageways, a biased closed safety cutoff valve, a pressure regulator and a manual valve in said common fuel passageway, manual means for opening said cutoff valve, electromagnetic means operative to hold said cutoff valve open when energized, a thermocouple junction operative to energize said electromagnetic means when heated by pilot flame, and means mounted in said body and responsive to the fuel pressure in said pilot burner passageway and operative to de-energize said electromagnetic means when the pressure drops below a predetermined minimum.

9. The fuel flow control device claimed in claim 8 in which said pressure responsive means comprises a pressure operated switch controlling circuit connections between said thermocouple junction and said electromagnetic means, said switch comprising a cup-like casing, a flexible diaphragm, a pair of spaced stationary contacts, a biasing spring, and a conductive bridging member connected to said diaphragm, said body member having a recess formed therein and said switch being mounted in said recess. 

1. In a fuel flow control device for gas burners, a body having an inlet, an outlet, and means forming a connecting passageway, a pilot burner, a biased closed safety cutoff valve controlling said passageway, manual means for opening said cutoff valve, electromagnetic means operative when energized to hold said cutoff valve open, an energizing circuit for said electromagnetic means including as a power source a thermocouple junction heated by said pilot burner, and a pressure responsive switch controlling said energizing circuit, said switch being responsive to gas pressure in said passageway and operative to complete said energizing circuit when the gas pressure is above a predetermined minimum and to break said energizing circuit when the gas pressure drops below said predetermined minimum.
 2. In a fuel flow control device for gas burners, a body having a valve chamber, an inlet and an inlet fuel pAssageway leading to said valve chamber, a main burner outlet, a pilot burner outlet, and main and pilot burner fuel passageways leading from said valve chamber to said outlets, a manual valve in said valve chamber controlling the flow through said main and pilot burner passageways, a biased closed safety cutoff valve in said valve chamber, manual means for opening said cutoff valve, electromagnetic means for holding said cutoff valve open, an energizing circuit for said electromagnetic means including a thermocouple junction heated by said pilot burner, and a pressure responsive switch controlling said energizing circuit, said switch being responsive to gas pressure in said pilot burner fuel passageway to complete said energizing circuit when the gas pressure is above a predetermined minimum and to break said energizing circuit when the pressure drops below said predetermined minimum.
 3. The fuel flow control device claimed in claim 2 in which the manual valve is operative to selectively permit the flow of fuel through said main and pilot burner fuel passageways.
 4. The fuel flow control device claimed in claim 3 which further includes a thermostatically actuated valve controlling said main burner fuel passageway.
 5. The fuel flow control device claimed in claim 2 in which said safety cutoff valve is positioned upstream from said manual valve.
 6. In a burner control system, a main burner, a pilot burner, a source of fluid fuel under pressure, thermostatic valve means for controlling the flow of fuel to the main burner, manual valve means for controlling the flow of fuel to both main and pilot burners, a biased closed safety cutoff valve controlling the flow to both main and pilot burners, manual means for opening said safety cutoff valve, means operative in response to flame at said pilot burner to hold said safety valve open, and pressure responsive means operative in response to a drop in fuel supply pressure below a predetermined minimum to render said safety cutoff valve holding means inoperative.
 7. The burner control system claimed in claim 6 which further includes pressure regulating means and in which said fuel pressure responsive means is responsive to the fuel pressure as regulated by said pressure regulating means.
 8. In a fuel flow control device for gas burners, a main burner and a pilot burner for igniting the main burner, a body member, a fuel inlet, a pilot burner outlet, and a main burner outlet in said body, said body being further provided with a pilot fuel passageway leading to said pilot burner outlet, a main burner fuel passageway leading to said main burner outlet and means forming a common fuel passageway connecting said inlet with said pilot and main burner fuel passageways, a biased closed safety cutoff valve, a pressure regulator and a manual valve in said common fuel passageway, manual means for opening said cutoff valve, electromagnetic means operative to hold said cutoff valve open when energized, a thermocouple junction operative to energize said electromagnetic means when heated by pilot flame, and means mounted in said body and responsive to the fuel pressure in said pilot burner passageway and operative to de-energize said electromagnetic means when the pressure drops below a predetermined minimum.
 9. The fuel flow control device claimed in claim 8 in which said pressure responsive means comprises a pressure operated switch controlling circuit connections between said thermocouple junction and said electromagnetic means, said switch comprising a cup-like casing, a flexible diaphragm, a pair of spaced stationary contacts, a biasing spring, and a conductive bridging member connected to said diaphragm, said body member having a recess formed therein and said switch being mounted in said recess.
 10. The burner control system claimed in claim 6 in which said means operative in response to pilot flame to hold said safety valve open comprises an electromagnetic device, a thermocouple junction adjacent the pilot burner, and Circuit connections, and in which said pressure responsive means comprises a pressure responsive switch controlling said circuit connections. 